As I wrote then:

“…when you’re archiving older data, it’s much more useful for the ZIP file date to reflect the date of its contents, since a few years from now you probably won’t care when you compressed the stuff, but rather how old the files inside are.”

With Mac OS 10.6 Snow Leopard, this same workflow can be set up to run as a service, accessible from the context menu in any Finder window. The process is essentially the same as described in the original post, with the following adjustments:

To create the workflow,

1. Open /Applications/Automator.app, select the Service template and click Choose.
2. At the top of the workflow area on the right side of the window, change the first list option to files or folders.
3. From the Finder library, drag the Create Archive action to the workflow area. In the action settings, specify the default name and location for the archive file. (I prefer the same name & folder as the input, but you can set the Options to prompt for this information when the workflow runs.)
4. From the Automator library, add the Run Shell Script action. Leave the Shell set to /bin/bash, set Pass input to as arguments, and replace the default script in the text box with zip -o "$@".

Save the workflow as a service, and you can run it on files & folders in the Finder by Control-clicking and choosing your new command from the Services submenu in the shortcut menu.

Note: To create a service that will set the date for existing ZIP files, skip Step 3 above.

But when you’re archiving older data, it’s much more useful for the ZIP file date to reflect the date of its contents, since a few years from now you probably won’t care when you compressed the stuff, but rather how old the files inside are.

Fortunately, the command line version of ZIP included in Mac OS X includes an option to do just this: zip -o, which according to the man page will

Set the “last modified” time of the zip archive to the latest (oldest) “last modified” time found among the entries in the zip archive

—which is just what we want.

But depending on your command line proficiency, you may find it a bit tedious to open up a terminal window and look up the command syntax every time you need to compress a file.

Enter Automator, one of the most useful (and unsung) productivity tools in OS X, which we’ll use to create a new Finder plug-in (or “workflow”).

To create the workflow, open /Applications/Automator.app and add three actions in this order:

1. From the Finder library, drag the Get Selected Finder Items action to the workflow area on the right side of the window.

2. Drag the Create Archive Finder action to the workflow area, placing it after the first action.

   (At this point, you can pre-specify a name and location for the archive file, or set the Options to prompt for this information when the workflow runs.)

3. From the Automator library, add the Run Shell Script action. Leave the Shell set to /bin/bash, set Pass input to as arguments, and replace the default script in the text box with zip -o "$@".

That’s it!—Save the workflow as a Finder Plug-in, and you can run it on files & folders in the Finder by Control-clicking and choosing your new command from the Automator submenu in the shortcut menu.